This invention relates to video signal processing apparatus and, more particularly, to apparatus which decreases the subjectively annoying effects of an extraneous signal on a video signal.
Video signals, when transmitted over open-wire lines for extended lengths, are subjected to unwanted interference. In addition, video signals in fringe areas of reception may be so weak that the ever-present Johnson or white noise may mask the actual picture element values in the receiving apparatus and create what is known as "snow" in the television art. In the fringe areas of reception, early receiving apparatus was frequently preceded by an amplifying device or booster which effectively decreased the bandwidth of the receiving apparatus in order to remove the effects of as much noise as possible. In so doing, these amplifiers also removed the high frequency energy present in the video signal and the resulting picture suffered from a noticeable decrease in horizontal resolution. The effects of the random noise, however, were reduced from a subjective standpoint.
In still other prior art apparatus of the type disclosed in U.S. Pat. No. 2,841,645 of July 1, 1958 to M. J. Stateman, the video signal is coupled through an amplitude gate whose transfer characteristics are determined by an output signal which has been delayed by a field or frame interval. If the input video signal is equal in amplitude to the delayed output signal, within the limits of some predetermined range, the input video signal is coupled without change to the output. If, however, the input video signal is outside of the predetermined range established by the output signal, the input video signal is clamped to either the upper or lower limit of the established range. As a result, the amplitude for any picture element is not permitted to change during any given frame interval by more than a threshold level. Any unwanted noise which is uncorrelated to the frame interval is effectively clipped so as to reduce its subjective effect on the picture. The Stateman apparatus works well for pictures that are stationary or for very slow movements within the picture. However, as the movements in the picture accelerate, the Stateman apparatus produces a streaking since picture element values are not able to change as rapidly as the moving objects in the picture demand.